Electronic microscope, focused ion beam, micro-focus X-ray tube, electron beam lithography machine, ion beam lithography machine and the like each play an important role in current micro- and nano-science and technology.
Electron source for generating electrons and ion source for generating ions (collectively referred to as particle source, which is usually formed of a metal wire) are key components in the above-mentioned imaging systems. The performance of the particle source is ultimately important to some critical characteristics of the imaging systems, such as resolution, or contrast.
The most common electron source is Schottky field emission electron source, which has advantages including a long life time over a conventional thermal electron source. However, this electron source also has some disadvantages, such as larger virtual source diameter, lower resolution and brightness, and uneven distribution of electron energy. Cold field emission electron source is currently considered as having a better performance in that it has a smaller virtual source diameter, and provides a higher brightness at high voltages than the Schottky field emission electron source. The brightness, however, would deteriorate dramatically at low voltages. This is not suitable for detection of many materials, particularly biological samples, which must be detected at a low voltage, otherwise would be damaged.
Gas field ion source has been introduced for many years. It has advantages, such as smaller virtual source diameter and uniform distribution of ion energy, and also disadvantages like lower beam current density. In addition, the size of tip is limited by micro- and nano-fabrication processes, and thus the divergence angle of an ion source is too large to be applied in a focused ion beam system. Currently, the most common ion source is liquid metal ion source which has a high beam current density but a larger virtual source diameter and uneven distribution of ion energy. Recently, with successful development of pyramid atom-level tip, it is possible for gas field ion source to be applied in focused ion beam systems. However, the shortcoming of a very low beam current density is still a restriction on application of the gas field ion source, especially in the field of micro- and nano-manufacture.
There is thus a need for a particle source capable of providing particle beam with high beam current density, small beam opening angle, reduced virtual source diameter and small energy spread. It is also desirable that an electron source could emit electron beams with good spatial coherence.